Aircraft are required to ground taxi between locations on airfields. An example is taxiing between a runway and the location (e.g. terminal gate) at which the aircraft's passengers are to board or disembark. Typically, such taxiing is achieved by using the thrust from the aircraft's engines to propel the aircraft forwards so that the landing gear wheels are caused to rotate. Since ground taxi speeds are necessarily relatively low, the engines must be run at a very low power. This means that there is a relatively high fuel consumption as a result of the poor propulsion efficiency at this low power. This leads to an increased level of both atmospheric and noise pollution locally around airports. Moreover, even when the engines are run at low power it is generally necessary to apply the wheel brakes to limit ground taxi speeds, leading to a high degree of brake wear.
Several autonomous ground taxi systems for driving the wheels while the aircraft is on the ground have been proposed in recent years. An example is disclosed in US2006/0065779, which proposes a powered nose aircraft wheel system. Prior art arrangements not restricted to nose landing gears are described in WO2011/023505 and WO2014/023939 which use an actuator to move a drive pinion in and out of driving engagement with a driven gear mounted to the wheel hub.
The autonomous ground taxi system derives its power from either a battery storage device or the auxiliary power unit (APU), which conventionally is a gas turbine generator but alternatively may be a fuel cell or similar electricity generator. When the aircraft is operating under the autonomous ground taxi system the main aircraft engines typically will be switched off. For aircraft with hydraulic steering and/or braking systems the hydraulic accumulator is typically driven by the main aircraft engines. When the main engines are switched off the steering/braking systems may require additional system redundancy to operate accommodating failure.